The present invention relates to wireless communications systems and methods, and more particularly, to apparatus and methods for synchronization and cell search in wireless communications systems.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a typical terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system 20 may include one or more radiotelephones (terminals) 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of a base station 26 is to handle radio communication between a cell 24 and mobile terminals 22. In this capacity, a base station 26 functions as a relay station for data and voice signals.
Those skilled in the art will appreciate that xe2x80x9ccellsxe2x80x9d may have configurations other than the omnidirectional cells 24 illustrated in FIG. 1. For example, the coverage areas conceptually illustrated as a hexagonally-shaped area served by a base station 26 may actually be subdivided into three sectors using separate directional antennas mounted at the base station 26, with the sector antenna having patterns extending in three different directions. Each of these sectors may in itself be considered a xe2x80x9ccell.xe2x80x9d As will be appreciated by those skilled in the art, other cell configurations are also possible, including, for example, overlaid cells, microcells, picocells and the like.
As illustrated in FIG. 2, a satellite 42 may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and terminals 23. The satellite conveys radiotelephone communications over duplex links 46 to terminals 23 and an earth station 44. The earth station 44 may in turn be connected to a public switched telephone network 34, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system 40 may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 48, each serving distinct geographical coverage areas 50 in the system""s service region. The coverage areas 50 serve a similar function to the cells 24 of the terrestrial cellular system 20 of FIG. 1.
Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels. As a practical matter well known to those skilled in the art, radiotelephone communications signals, being modulated waveforms, typically are communicated over predetermined frequency bands in a spectrum of carrier frequencies. In a typical FDMA system, each of these discrete frequency bands serves as a channel over which cellular radiotelephones communicate with a cell, through the base station or satellite serving the cell.
The limitations on the available frequency spectrum present several challenges as the number of subscribers increases. Increasing the number of subscribers in a cellular radiotelephone system requires more efficient utilization of the limited available frequency spectrum in order to provide more total channels while maintaining communications quality. This challenge is heightened because subscribers may not be uniformly distributed among cells in the system. More channels may be needed for particular cells to handle potentially higher local subscriber densities at any given time. For example, a cell in an urban area might conceivably contain hundreds or thousands of subscribers at any one time, easily exhausting the number of channels available in the cell.
For these reasons, conventional cellular systems employ frequency reuse to increase potential channel capacity in each cell and increase spectral efficiency. Frequency reuse involves allocating frequency bands to each cell, with cells employing the same frequencies geographically separated to allow radiotelephones in different cells to simultaneously use the same frequency without interfering with each other. By so doing, many thousands of subscribers may be served by a system having only several hundred allocated frequency bands.
Another technique which can further increase channel capacity and spectral efficiency is the use of time division multiple access (TDMA). A TDMA system may be implemented by subdividing the frequency bands employed in conventional FDMA systems into sequential time slots. Communications over a frequency band typically occur on a repetitive TDMA frame structure that includes a plurality of time slots. Examples of systems employing TDMA are those conforming to the dual analog/digital IS-54B standard employed in the United States, in which each of the frequency bands of the traditional analog cellular spectrum are subdivided into 3 time slots, and systems conforming to the GSM standard, which divides each of a plurality of frequency bands into 8 time slots. In these TDMA systems, each user communicates with the base station using bursts of digital data transmitted during the user""s assigned time slots.
Yet another technique for potentially increasing system capacity is to employ xe2x80x9cspread spectrumxe2x80x9d code division multiple access (CDMA) techniques. In a system employing spread spectrum techniques, a channel may be defined by modulating a data-modulated carrier signal by a unique spreading code, i.e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates. Data may be recovered from the transmitted signal by demodulating the signal using the same spreading code. Because the transmitted signal is spread across a wide bandwidth, spread spectrum communications can be less vulnerable to coherent noise sources which might xe2x80x9cjamxe2x80x9d other communications signals. The use of unique spreading codes for channels allows several users to effectively share the same bandwidth without undue interference.
Conventional spread-spectrum communications systems commonly use so-called xe2x80x9cdirect sequencexe2x80x9d spread spectrum modulation. In direct sequence modulation, a data-modulated carrier is directly modulated by a spreading code or sequence before being transmitted in a communications medium, e.g., an air interface. The spreading code typically includes a sequence of xe2x80x9cchipsxe2x80x9d occurring at a chip rate that typically is much higher than the bit rate of the data being transmitted.
A direct sequence spread spectrum receiver typically includes a local sequence generator that locally produces a replica of a spreading sequence. This locally generated sequence is used to recover information from a transmitted spread spectrum signal that is modulated according to the same spreading sequence. Before information in a transmitted signal can be recovered, however, the locally generated spreading sequence typically must be synchronized with the spreading sequence that modulates the transmitted signal.
Synchronization of terminals is commonly achieved by transmitting a synchronization signal in each cell that a terminal can acquire to obtain a timing reference for synchronizing its de-spreading operations. For example, in an IS-95 compliant system, a xe2x80x9cpilot channelxe2x80x9d including a fixed carrier modulated by a known sequence is transmitted in each cell of the system, with a respective timing offset applied in a respective cell. In other systems, such as in systems using wideband CDMA techniques, a common synchronization code (or a code from a common set of synchronization codes) is embedded within time slots defined in data frames of a downlink channel at known locations. This sequence, sometimes referred to as a xe2x80x9cfirst synchronization codexe2x80x9d (FSC) or a xe2x80x9cprimary synchronization codexe2x80x9d (PSC), is detected by a terminal and used to aid the terminal in determining slot timing.
As a terminal operates in a wireless cellular system, it typically seeks to identify new cells, usually neighboring cells, with which it can communicate should the signal quality of the link between the terminal and the cell with which it currently is communicating become degraded. For example, a terminal actively engaged in a call via a base station serving one cell typically needs to identify other base stations to which the call can be handed over as the terminal moves through the system. Terminals emerging from a sleep mode may also engage in cell search operations, as the synchronization signals of a set of xe2x80x9ccandidate cellsxe2x80x9d identified by the terminal before going to sleep may have degraded or even disappeared while the terminal was asleep.
The above-described synchronization signals are commonly used in such cell search operations. For example, in proposed WCDMA systems, a base station transmits over a downlink channel according to a cell-specific xe2x80x9clongxe2x80x9d (e.g., 40,960 chip) scrambling code that serves to identify the cell. The scrambling codes are typically divided into groups to make cell search more efficient. To identify the group to which a cell belongs, a base station typically transmits a secondary synchronization code (SSC) associated with a group to which the cell belongs, in parallel with the primary synchronization code. The SSC also typically has a period of one frame, and thus provides a reference for determining frame boundaries. In newly proposed WCDMA systems, the functions of the SSC are incorporated into the PSC by transmitting the PSC in one of a set of predetermined patterns within a frame of a synchronization signal, with the pattern representing the scrambling code group to which the cell belongs.
To perform a cell search in a system in which the PSC is transmitted at a fixed position at the beginning of a slot of a data frame, a terminal first identifies a candidate slot boundaries by correlating a received signal with the common primary synchronization code (FSC or PSC) over a predetermined time interval, e.g., 30 msec. The correlations generated are examined for peaks that indicate the presence of the PSC. Once candidate slot boundaries have been identified, a second stage is initiated in which the terminal correlates the received signal with each of the SSCs using the candidate slot boundaries. If a sufficient correlation is found between the received signal and one of the SSCs, indicating a likelihood that the cell associated with the SSC uses a long code that is a member of the group associated with the SSC, the terminal can then correlate the received signal with a relatively small set of long codes. In this manner, a cell associated with a synchronization signal can be identified in an efficient manner, without correlating a received signal with all possible long codes. Such a cell search procedure is described in detail in Version 1.0 of the xe2x80x9cSpecifications of Air-Interface for 3G Mobile System,xe2x80x9d published by Association of Radio Industries and Businesses (ARIB), Jan. 14, 1999, and in xe2x80x9cPerformance and Complexity of Techniques for Achieving Fast Sector Identification in an Asynchronous CDMA System,xe2x80x9d by Ostberg et al., published in Proceedings of the 1998 Wireless Multimedia Conference, Japan, November 1998.
The above-described cell search procedure can be adversely affected by characteristics of the radio propagation environment. For example, as the same common synchronization sequence is typically transmitted by base stations serving all of the cells of the system, the synchronization signals transmitted by the stations may interfere with one another, which can make it difficult to identify slot boundaries for a particular synchronization signal associated with a particular cell. This problem may be exacerbated in dispersive channels, where multipath components may cause additional interference.
In light of the foregoing, it is an object of the present invention to provide improved methods and apparatus for determining timing of a synchronization signal.
It is another object of the present invention to provide improved cell search methods and apparatus.
It is another object of the present invention to provide improved cell search methods and apparatus suitable for use in wideband CDMA (WCDMA) systems.
These and other objects, features and advantages are provided according to the present invention by systems and methods in which a received communications signal representing a combination of synchronization signals transmitted for cells in a wireless communications system is correlated with a common synchronization code, such as the first synchronization code (FSC) or primary synchronization code (PSC) used in a WCDMA system. A component associated with a known synchronization signal, such as a synchronization signal transmitted by a base station serving a cell maintained in a xe2x80x9cneighborxe2x80x9d list at the receiving terminal, is canceled from the correlation to produce an interference canceled correlation that may be used for determining timing, e.g., slot boundaries, of a synchronization signal. The timing information may be further used to determining the identity of a cell for which the synchronization signal is transmitted, for example, by providing a timing basis for determining a scrambling group code or secondary synchronization code (SSC), which in turn is used to guide detection of a cell-specific scrambling (long) code. The interference cancellation and timing determination techniques of the present invention may also be advantageous, for example, for determining timing of synchronization signals such as the pilot channel signals broadcast in IS-95 systems.
The methods and apparatus of the present invention offer improved timing determination and cell search techniques that are potentially more efficient than conventional search techniques. Canceling signal components associated with known synchronization signals can aid in detecting slot boundaries of a desired synchronization signal, and can thus speed a cell search process. According to additional aspects of the present invention, the cells identified in a cell search process can in turn provide increase knowledge about potentially interfering synchronization signals that may be used to enhance an interference cancellation process.
In particular, according to an aspect of the present invention, a received communications signal representing a combination of synchronization signals transmitted in cells of a wireless communications system is processed. The received communications signal is correlated with a common synchronization code to produce a synchronization detection signal. A component of the synchronization detection signal associated with a known synchronization signal is canceled from the synchronization detection signal to produce an interference-canceled synchronization detection signal. Timing of a synchronization signal is determined from the interference-canceled synchronization detection signal.
According to one aspect of the present invention, interference cancellation is achieved by generating a correlation of an estimated received known synchronization signal with the common synchronization code, canceling the correlation of the estimated received known synchronization signal with the common synchronization code from the synchronization detection signal to produce the interference-canceled synchronization detection signal. Timing of a synchronization signal may then be determined by accumulating the interference-canceled synchronization detection signal over a time interval, detecting a peak in the accumulated interference-canceled synchronization detection signal, and determining timing of a synchronization signal from the detected peak. The correlation of an estimated received known synchronization signal with the common synchronization code be generated by processing filtering a representation of the known synchronization signal with an estimate of a channel over which the known synchronization signal is transmitted to produce an estimated received known synchronization signal, and correlating the estimated received known synchronization signal with the synchronization code.
According to another aspect of the present invention, interference cancellation is achieved by accumulating the synchronization detection signal over a time interval, and identifying a peak in the accumulated synchronization detection signal not associated with a known synchronization signal. Timing of a synchronization signal is determined from the identified peak. Identification of the peak may be conditioned upon the peak meeting a predetermined criterion.
The known synchronization signal may include a synchronization signal associated with a previously identified cell, such as a synchronization signal associated with a cell with which the terminal is currently communicating over a traffic channel. For example, interference cancellation may be preceded by identification of a set of synchronization signals associated with a set of candidate cells. Interference cancellation may then include canceling a component of the synchronization detection signal corresponding to a synchronization signal associated with a cell of the set of candidate cells from the synchronization detection signal to produce an interference-canceled synchronization detection signal.
In related aspects, the set of known synchronization signals may be identified by receiving a communications signal from the communications medium, identifying a synchronization signal in the received communications signal, and identifying a cell with which the identified synchronization signal is associated. The identified cell may be added to the set of candidate cells if the identified synchronization signal associated with the identified cell meets a predetermined criterion.
A respective synchronization signal may include a portion encoded according to the common synchronization code, such as the first search code (FSC) or primary search code (PSC) transmitted in a wideband CDMA (WCDMA) system. A common synchronization signal may also be transmitted in each of the cells over a pilot channel, such as conventionally done in IS-95 systems.
According to yet another aspect of the present invention, the correlating, canceling and determining are performed in response to the terminal awaking from a sleep mode. For example, a terminal may awaken from a sleep mode, receive a communications signal and evaluating a set of candidate cells based on the received communications signal. The received signal may then be correlated with the common synchronization code to produce a synchronization detection signal if the evaluated candidate cells fail to meet a predetermined criterion. The synchronization detection signal is accumulated over a time interval, and a peak in the accumulated synchronization detection signal not associated with a known synchronization signal associated with one of the candidate cells is identified. Timing for a synchronization signal may then be determined from the identified peak, and the determined timing may be used in identifying a new candidate cell associated with a synchronization signal associated with the identified peak.
Terminal apparatus operative to perform the above-described functions are also described. In one embodiment, a terminal includes an apparatus including a first correlator, e.g., a sliding correlator, operative to correlate a received communications signal with a common synchronization code to produce a synchronization detection signal. An interference canceler is responsive to the first correlator and operative to cancel a component of the synchronization detection signal associated with a known synchronization signal from the synchronization detection signal to produce an interference-canceled synchronization detection signal. A timing determiner operative to determine timing of a synchronization signal from the interference-canceled synchronization detection signal.
In another embodiment according to the present invention, the interference canceler includes a received known synchronization signal estimator that is operative to generate an estimated received known synchronization signal. A second correlator is responsive to the received known synchronization signal estimator and operative to generate a correlation of an estimated received known synchronization signal with the common synchronization code. A canceler is responsive to the first correlator and to the second correlator and operative to cancel the correlation of the estimated received known synchronization signal with the common synchronization code from the synchronization detection signal to produce the interference-canceled synchronization detection signal. The timing determiner includes an accumulator responsive to the canceler and operative to accumulate the interference-canceled synchronization detection signal over a time interval. A peak detector is responsive to the accumulator and operative to detect a peak in the accumulated interference-canceled synchronization detection signal.
In yet another embodiment according to the present invention, the interference canceler includes an accumulator responsive to the first correlator and operative to accumulate the synchronization detection signal over a time interval. A peak detector is responsive to the accumulator and operative to detect a peak in the accumulated synchronization detection signal not associated with a known synchronization signal. The timing determiner is responsive to the peak detector and operative to determine timing of a synchronization signal from the detected peak.
Improved methods and apparatus for determining timing of synchronization signals and for identifying cells associated with synchronization signals may thereby be provided.